There exist communication technologies using a high-frequency band between 3 and 30 MHz (see, for example, Japanese Laid-Open Patent Publication No. 2005-333291 and Japanese Laid-Open Patent Publication No. 2003-234683). A radio signal in the high-frequency band is reflected by the ionosphere and the surface of the earth and can travel a long distance, and is therefore used for long-distance communications.
However, the condition of the ionosphere changes as time passes and depending on the condition, the ionosphere allows a radio signal to pass therethrough, attenuates a radio signal, or reflects a radio signal. Also, the ionosphere behaves differently toward radio signals with different frequencies.
For the above reasons, communications using the high-frequency band are liable to be affected by the ever-changing ionosphere and are unstable. Accordingly, it is desired to improve the transmission quality of communications using the high-frequency band.